1. Technical Field of the Invention
The present invention relates to a liquid crystal display apparatus and particularly, to its structure for widening a visual field angle.
2. Description of the Prior Art
Conventionally, a liquid crystal display apparatus utilizing twisted nematic (TN) liquid crystal is widely known. This equipment, however, has a disadvantage that the tone of color depends upon an angle of view due to the uniform orientation of the TN molecular in a pixel under an applied voltage. In order to reduce the visual angle dependence, the orientation of the liquid crystal molecules is divided by an oblique electric field by utilizing an aperture formed in each pixel electrode or common electrode. According to the above-mentioned orientation division, the image quality is improved, because each of the divided orientations compensates with each other.
The orientation division as disclosed in JP10-20323 A (1998) is further explained, referring to a cross sectional view as shown in FIG. 4 and a plan view as shown in FIG. 5.
Aperture 12 is formed in pixel electrode 13. Further control electrode 14 is formed at the position of aperture 12. Concretely, control electrode 14 may be formed in the layer of pixel electrode, or in other layers over or under aperture 12. Further, pixel electrode 13 is insulated from control electrode 14 to apply different voltages. The oblique electric field is generated by the application of voltage to control electrode 14. Therefore, the orientation of liquid crystal molecules 18 is inclined into various directions. The orientation as shown in FIG. 4 is an exemplary orientation at the middle position in the direction of the layer thickness of the liquid crystal. Then, a small quantity of a monomer or olygomer added in liquid crystal 18 is polymerized by ultra violet (UV) exposure under the applied electric field. The generated polymer 19 fixes the orientation of liquid crystal molecules 18 even after switching off the voltage. The orientation of liquid crystal molecules 18 is varied by the voltage applied to pixel electrode, because polymer content in the liquid crystal layer is so small that only the initial orientation is fixed. Thus, the angular dependence of the tone of color can be reduced.
FIG. 5 is a plan view of an example as shown in JP 10-20323 A (1998). Control electrode 14 and gate electrode 3 are formed in the same layer in this example. Pixel electrode 13 can not be connected electrically with a driving circuit in the same plane as control electrode 14, if pixel electrode 13 is divided into parts completely by control electrode 14 in the same layer. Accordingly, in the above-mentioned example, control electrode 14 should be formed in a layer different from pixel electrode 13, because the same voltage should be applied to each of the divided parts in a pixel. On the contrary, pixel electrode 13 should be connected as shown in FIG. 5 in spite of the introduction of aperture 12, if control electrode 14 and pixel electrode 13 must be formed in the same layer.
However, the oblique electric field is not generated at several portions over pixel electrode 13, because pixel electrode is connected partially ,or divided incompletely there. In this case, the displayed tone varies from pixel to pixel, depending upon the visual angle, or the angle at which the display is seen.
Another technology as shown in FIG. 6, concerning the aperture in the pixel electrode is disclosed in JP 7-199190 A (1995). As shown in a cross sectional view in FIG. 6, aperture 27 is formed in pixel electrode 26 on substrate 1. Further, orientation control electrode 28 is formed near pixel electrode 26. The arrangement of pixel electrode 26 and orientation control electrode 28 is shown in a plan view in FIG. 7. Pixel electrode is surrounded by orientation control electrode 28 in order to regulate the direction of the electric field at the edge of pixel electrode 26. The load of the driving circuit for the display apparatus as shown in FIGS. 6 and 7 becomes heavy, because voltages are supplied for not only pixel electrode 26 and common electrode 15, but also orientation control electrode 28. Further, the connecting portions are indispensable in order to apply the same voltage to each of the divided portions. Therefore, the display apparatus as disclosed in JP 7-199190 A (1995) has the same disadvantage as that disclosed in JP 10-20323 A (1998).